ARI SHAPIRO, BYLINE: The three-day marathon at the U.S. Supreme Court continues today. The court will hold its second day of hearings on President Obama's health care law. Today, the lawyers and justices will spar over whether the individual mandate is constitutional. That's a requirement that everyone carry health insurance, and it's a central tenant of the law.

GREENE: The first day of arguments yesterday was more of a gatekeeper, asking whether the justices have the authority to consider the essential questions in the case at all.

NPR's Ari Shapiro started yesterday on the steps of the courthouse.

(SOUNDBITE OF PROTESTS)

SHAPIRO: Dueling protests outside of the Supreme Court. On the one hand you have protesters in favor of the health care law, marching with signs that say Don't Deny My Health Care, Protect the Law, clearly mass printed. Everybody's got the same sign. And then there's a smaller group of people wearing Tea Party Patriots T-shirts with handmade signs. One of them says Unlawful, Lacks Consent of the Governed. And while the pro health care law people are chanting we love Obamacare, the anti health care law people are chanting we love the Constitution, all at the steps of the Supreme Court as the sun rises behind the building.

Inside, things were far more somber.

(SOUNDBITE OF SUPREME COURT HEARING)

ROBERT LONG: We will hear argument this morning...

SHAPIRO: The day's question was whether the Supreme Court should be hearing this case at all. An 1867 law called the Anti-Injunction Act says people can only go to court challenging a tax after they have paid it. Under the health care law, it's another three years until anybody pays a penalty for lacking health insurance. If the penalty is a tax, then it's another three years until the court can hear this case.

Attorney Robert Long said the penalty is a tax, so all the hubbub outside the court is premature.

(SOUNDBITE OF SUPREME COURT HEARING)

LONG: Congress provided that penalties are included in taxes for assessment purposes.

SHAPIRO: The justices appointed Long to make this argument, since neither side in the case agrees with it. Still, the justices gave Long a hard time. They threw one counter-example after another at him. Eventually Justice Sonia Sotomayor said...

(SOUNDBITE OF SUPREME COURT HEARING)

JUSTICE SONIA SOTOMAYOR: From all the questions here, I count at least four cases in the court's history where the court has accepted a waiver by the Solicitor General and reached a tax issue.

LONG: Justice Stephen Breyer was equally dubious.

(SOUNDBITE OF SUPREME COURT HEARING)

JUSTICE STEPHEN BREYER: Now, here Congress has nowhere used the word tax. What it says is penalty. Moreover, this is not in the Internal Revenue Code but for purposes of collection. And so why is this a tax?

LONG: After 30 minutes, it was Solicitor General Don Verilli's turn. Back when this case was its earliest stages, the Obama administration argued that the penalty was a tax. Then they wanted the courts to hold off. But they've changed their minds and now they want the court to get to the meat of it. Here's Verilli.

(SOUNDBITE OF SUPREME COURT HEARING)

DON VERILLI: This case presents issues of great moment, and the Anti-Injunction Act does not bar the court's consideration of those issues.

JUSTICE ELENA KAGAN: Suppose a person does not purchase insurance, a person who is obligated to do so under the statute doesn't do it, pays the penalty instead, and that person finds herself in a position where she's asked the question, have you ever violated any federal law - would that person have violated a federal law?

VERILLI: No. Our position is that person should give the answer no.

KAGAN: And that's because...

VERILLI: That if they don't pay the tax, they've violated a federal law.

KAGAN: But as long as they've paid the penalty...

BREYER: If they've paid the tax, then they're in compliance with the law.

Why do you keep saying tax?

(SOUNDBITE OF LAUGHTER)

SHAPIRO: That was Justice Breyer, correcting Verilli's slip of the tongue as he kept referring to the penalty as a tax.

Finally, attorney Greg Katsas stepped up. He represents a group of states and businesses challenging the health care law. But like the Obama administration, he wants the justices to get to the heart of the argument now.

(SOUNDBITE OF SUPREME COURT HEARING)

GREG KATSAS: The purpose of this lawsuit is to challenge a requirement - a federal requirement - to buy health insurance. That requirement itself is not a tax. And for that reason alone, we think the Anti-Injunction Act doesn't apply.

SHAPIRO: The justices seemed to agree, which would free them up to get to the more substantive questions they'll be delving into today.